Entity Limits?

Hello, few days ago when I was playing, I found a skeleton spawner. I wanted to make old school farming with water and signs. I found out that it won't work so I used soul sand on the bottom and removed signs. It works just fine but since on dropping place is more than approx. 25+ skeletons, every new one dies. Is there some entity limit that was added, that makes them die after dropping from high place? I didn't played Minecraft since 1.8 so I am confused by all of this

There's a new 'rule' called 'maxEntityCramming' that prevents too many mobs from being in the same place, IIRC this is defaulted to 24
According to the Wiki this is

The maximum number of other pushable entities a mob or player can push, before taking 3 () suffocation
damage per half-second.
Setting to 0 disables the rule. Damage affects survival-mode or adventure-mode players, and all mobs but bats.

So I'm guessing that your 'drop place' is probably a 1x1 block area meaning you can only have a maximum of 24 Mobs in this space & any additional mobs that 'fall' into this place will cause the older mob on the block to start getting damage & eventually die

any additional mobs that 'fall' into this place will cause the older mob on the block to start getting damage & eventually die

Note quite accurate. When a mob exceeds maxCramming, all the mobs that it touches take damage equally until enough mobs die to drop back below maxCramming. This will not kill all the mobs at first, but the longer you let it go on the more mobs that will die at a time being that they're all creeping slowly towards 0 health.

For example, if the rule is set at 24 mobs and mob 25 falls in you could end up with all 25 mobs dying. You can get around this by using minecarts, they are classed as entities as well but they don't have any durability (so they last forever).

If you are trying to do an xp farm, though, you will need to build a bigger kill chamber so that mobs have more room to spread out.

all the mobs that it touches take damage equally until enough mobs die to drop back below maxCramming

I stand corrected... considering I said if you had a 1x1 drop zone I should've realised the 25th that drops in would be touching all the other mobs so it sort of makes sense that all of them would get damage and not just the oldest mob